9,434 research outputs found
Viscous effects on a vortex wake in ground effect
Wake vortex trajectories and strengths are altered radically by interactions with the ground plane. Prediction of vortex strength and location is especially important in the vicinity of airports. Simple potential flow methods have been found to yield reasonable estimates of vortex descent rates in an otherwise quiescent ambient background, but those techniques cannot be adjusted for more realistic ambient conditions and they fail to provide satisfactory estimates of ground-coupled behavior. The authors have been involved in a systematic study concerned with including viscous effects in a wake-vortex system which is near the ground plane. The study has employed numerical solutions to the Navier-Stokes equations, as well as perturbation techniques to study ground coupling with a descending vortex pair. Results of a two-dimensional, unsteady numerical-theoretical study are presented in this paper. A time-based perturbation procedure has been developed which permits the use of analytical solutions to an inner and outer flow domain for the initial flow field. Predictions have been compared with previously reported laminar experimental results. In addition, the influence of stratification and turbulence on vortex behavior near the ground plane has been studied
Automatic Lumbar Vertebrae Segmentation in Fluoroscopic Images via Optimised Concurrent Hough Transform
Low back pain is a very common problem in the industrialised countries and its associated cost is enormous. Diagnosis of the underlying causes can be extremely difficult. Many studies have focused on mechanical disorders of the spine. Digital videofluoroscopy (DVF) was widely used to obtain images for motion studies. This can provide motion sequences of the lumbar spine, but the images obtained often suffer due to noise, exacerbated by the very low radiation dosage. Thus determining vertebrae position within the image sequence presents a considerable challenge. In this paper, we show how our new approach can automatically detect the positions and borders of vertebrae concurrently, relieving many of the problems experienced in other approaches. First, we use phase congruency to relieve difficulty associated with threshold selection in edge detection of the illumination variant DVF images. Then, our new Hough transform approach is applied to determine the moving vertebrae, concurrently. We include optimisation via a genetic algorithm as without it the extraction of moving multiple vertebrae is computationally daunting. Our results show that this new approach can indeed provide extractions of position and rotation which appear to be of sufficient quality to aid therapy and diagnosis of spinal disorders
Lumbar Spine Location in Fluoroscopic Images by Evidence Gathering
Low back pain (LBP) is a very common problem and lumbar segmental instability is one of the causes. It is important to investigate lumbar spine movement in order to understand instability better and as an aid to diagnosis. Digital videofluoroscopy provides a method of quantifying the motion of individual vertebrae, but due to the relatively poor image quality, it is difficult and time consuming to locate landmarks manually, from which the kinematics can be calculated. Some semi-automatic approaches have already been developed but these are still time consuming and require some manual interaction. In this paper we apply the Hough transform (HT) to locate the lumbar spinal segments automatically. The HT is a powerful tool in computer vision and it has good performance in noise and partial occlusion. A recent arbitrary shape representation avoids problems inherent with tabular representations in the generalised HT (GHT) by describing shapes using a continuous formulation. The target shape is described by a set of Fourier descriptors, which vote in an accumulator space from which the object parameters of translation (including the x and y direction), rotation and scale can be determined. At present, this algorithm has been applied to the images of lumbar spine, and has been shown to provide satisfactory results. Further work will concentrate on reducing the computational time for real-time application, and on approaches to refine information at the apices, given initialisation by the new HT method
Second coefficient of viscosity in air
Acoustic attenuation measurements in air were analyzed in order to estimate the second coefficient of viscosity. Data over a temperature range of 11 C to 50 C and at relative humidities between 6 percent and 91 percent were used. This analysis showed that the second coefficient of viscosity varied between 1900 and 20,000 times larger than the dynamic or first coefficient of viscosity over the temperature and humidity range of the data. In addition, the data showed that the molecular relaxation effects, which are responsible for the magnitude of the second coefficient of viscosity, place severe limits on the use of time-independent, thermodynamic equations of state. Compressible flows containing large streamwise velocity gradients, like shock waves, which cause significant changes in particle properties to occur during time intervals shorter than hundredths of seconds, must be modeled using dynamic equations of state. The dynamic model approach is described briefly
Bidding Behavior in Competing Auctions: Evidence from eBay
Much of the existing auction literature treats auctions as running independently of one another, with each bidder choosing to participate in only one auction. However, in many online auctions, a number of substitutable goods are auctioned concurrently and bidders can bid on several auctions at the same time. Recent theoretical research shows how bidders can gain from the existence of competing auctions, the current paper providing the first empirical evidence in support of competing auctions theory using online auctions data from eBay. Our results indicate that a significant proportion of bidders do bid across competing auctions and that bidders tend to submit bids on auctions with the lowest standing bid, as the theory predicts. The paper also shows that winning bidders who cross-bid pay lower prices on average than winning bidders who do not.Competing Auction, Cross-Bidding, Auction Empirics
`Sinking' in a bed of grains activated by shearing
We show how a weak force, , enables intruder motion through dense granular
materials subject to external mechanical excitations, in the present case
stepwise shearing. A force acts on a Teflon disc in a two dimensional system of
photoelastic discs. This force is much smaller than the smallest force needed
to move the disc without any external excitation. In a cycle, material +
intruder are sheared quasi-statically from to , and
then backwards to . During various cycle phases, fragile and jammed
states form. Net intruder motion, , occurs during fragile periods
generated by shear reversals. per cycle, e.g. the quasistatic rate
, is constant, linearly dependent on and . It vanishes as,
, with and ,
reflecting the stiffening of granular systems under shear as . The intruder motion induces large scale grain circulation. In the
intruder frame, this motion is a granular analogue to fluid flow past a
cylinder, where is the drag force exerted by the flow.Comment: 4 pages, 5 figures letter with supplementarie
Force and Mass Dynamics in Non-Newtonian Suspensions
Above a certain solid fraction, dense granular suspensions in water exhibit
non-Newtonian behavior, including impact-activated solidification. Although it
has been suggested that solidification depends on boundary interactions,
quantitative experiments on the boundary forces have not been reported. Using
high-speed video, tracer particles, and photoelastic boundaries, we determine
the impactor kinematics and the magnitude and timings of impactor-driven events
in the body and at the boundaries of cornstarch suspensions. We observe mass
shocks in the suspension during impact. The shockfront dynamics are strongly
correlated to those of the intruder. However, the total momentum associated
with this shock never approaches the initial impactor momentum. We also observe
a faster second front, associated with the propagation of pressure to the
boundaries of the suspension. The two fronts depend differently on the initial
impactor speed, , and the suspension packing fraction. The speed of the
pressure wave is at least an order of magnitude smaller than (linear)
ultrasound speeds obtained for much higher frequencies, pointing to complex
amplitude and frequency response of cornstarch suspensions to compressive
strains
Assessment of the potential impacts of plant traits across environments by combining global sensitivity analysis and dynamic modeling in wheat
A crop can be viewed as a complex system with outputs (e.g. yield) that are
affected by inputs of genetic, physiology, pedo-climatic and management
information. Application of numerical methods for model exploration assist in
evaluating the major most influential inputs, providing the simulation model is
a credible description of the biological system. A sensitivity analysis was
used to assess the simulated impact on yield of a suite of traits involved in
major processes of crop growth and development, and to evaluate how the
simulated value of such traits varies across environments and in relation to
other traits (which can be interpreted as a virtual change in genetic
background). The study focused on wheat in Australia, with an emphasis on
adaptation to low rainfall conditions. A large set of traits (90) was evaluated
in a wide target population of environments (4 sites x 125 years), management
practices (3 sowing dates x 2 N fertilization) and (2 levels). The
Morris sensitivity analysis method was used to sample the parameter space and
reduce computational requirements, while maintaining a realistic representation
of the targeted trait x environment x management landscape ( 82 million
individual simulations in total). The patterns of parameter x environment x
management interactions were investigated for the most influential parameters,
considering a potential genetic range of +/- 20% compared to a reference. Main
(i.e. linear) and interaction (i.e. non-linear and interaction) sensitivity
indices calculated for most of APSIM-Wheat parameters allowed the identifcation
of 42 parameters substantially impacting yield in most target environments.
Among these, a subset of parameters related to phenology, resource acquisition,
resource use efficiency and biomass allocation were identified as potential
candidates for crop (and model) improvement.Comment: 22 pages, 8 figures. This work has been submitted to PLoS On
A New Look at Physical Layer Security, Caching, and Wireless Energy Harvesting for Heterogeneous Ultra-dense Networks
Heterogeneous ultra-dense networks enable ultra-high data rates and ultra-low
latency through the use of dense sub-6 GHz and millimeter wave (mmWave) small
cells with different antenna configurations. Existing work has widely studied
spectral and energy efficiency in such networks and shown that high spectral
and energy efficiency can be achieved. This article investigates the benefits
of heterogeneous ultra-dense network architecture from the perspectives of
three promising technologies, i.e., physical layer security, caching, and
wireless energy harvesting, and provides enthusiastic outlook towards
application of these technologies in heterogeneous ultra-dense networks. Based
on the rationale of each technology, opportunities and challenges are
identified to advance the research in this emerging network.Comment: Accepted to appear in IEEE Communications Magazin
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